Liquid crystal display device

ABSTRACT

It is an object of the present invention to provide a liquid crystal display device in which a liquid crystal layer may form a stable transparent state and, in addition, which provides for a black display with an excellent viewing angle characteristic, as well as a high contrast ratio and a high response speed in liquid crystal display. The present invention is directed to a liquid crystal display device comprising an electrode and a liquid crystal layer between substrates, said liquid crystal layer consists of a liquid crystalline material containing liquid crystal molecules and fine particles, and is optically isotropic when the voltage applied to the electrode is lower than a threshold value and undergoes optical transition due to change in the arrangement of the liquid crystal molecules when the applied voltage is not lower than the threshold value.

REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of application Ser. No. 10/929,725 filedon Aug. 31, 2004 now U.S. Pat. No. 7,463,328.

This Nonprovisional application claims priority under 35 U.S.C §119(a)on Patent Application No.2003-330905 filed in Japan on Sep. 24, 2003,No.2004-117102 filed in Japan on Apr. 12, 2004, No.2004-242338 filed inJapan on Aug. 23, 2004 and No.2005-085018 filed in Japan on Mar. 23,2005, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display device. Moreparticularly, the invention relates to a liquid crystal display devicecomprising an electrode and a liquid crystal layer both being formedbetween a pair of spaced-apart substrates which are placed oppositelyeach other.

DESCRIPTION OF THE RELATED ART

The liquid crystal display device is a display means utilizingcharacteristics of liquid crystal molecules, and is in broad use fordisplaying information and images. The liquid crystal display device ofthe type which is most prevalently in use at the present time is TN-LCD(twisted nematic liquid crystal display). Above all else, thefabrication technology for TN-TFT (thin film transistor)-LCD has made adramatic progress in recent years, even to the extent that the devicetoday is surpassing the CRT (cathode-ray tube) in front contrast ratioand color reproducibility. In regard to liquid crystal display devicessuch as TN-LCD, there is a demand for improving the narrow viewing angleof displays, which limits the scope of application of the devices, tothereby broaden their applicability taking various advantages of liquidcrystal displays.

For the purpose of improving such characteristics of liquid crystaldisplays, an LCD of the type called IPS (in-plane switching) has beenproposed but when the IPS system is adopted, the response speeddecreases because liquid crystal molecules are moved in a lateraldirection by an electric field. Moreover, for improving the viewingangle characteristic of the TN type, a VA (vertical alignment) systemhas been proposed. However, when the VA system is used, a black displayappears whitish when the viewing angle is shifted from the front, forexample, so that the contrast ratio is sacrificed although not so muchas in the case of the TN type.

Regarding new modes of liquid crystals, a phase diagram of a liquidcrystal microemulsion has been disclosed for a system comprising amixture of lyotropic liquid crystals and thermotropic liquid crystals(for example, refer to Yamamoto, Jun and one other: “Dynamics of LiquidCrystal Microemulsions”, Lecture Notes for the 1999 Seminar of JapaneseLiquid Crystal Society, Japanese Liquid Crystal Society, Sep. 29 to Oct.1, 1999, pages 318 to 319). In this publication, it is described that ina system comprising a mixture of a reversed micelle phase of a DDABaqueous solution and a nematic phase of a 5CB, notwithstanding the localformation of nematic orientational orders within a certainconcentration-temperature range, macroscopically, an isotropic phase wasformed and this has been named a transparent nematic phase. It isreported that in this transparent nematic phase, liquid crystalmolecules are forced into vertical alignment on the surface of thereversed micelle of the emulsion (a), and because the distance betweenthe micelles is sufficiently shorter than the visible light wavelength,the system shows macroscopically isotropic. The phase diagram of aliquid crystal microemulsion is shown in FIG. 6. In the diagram, (A),(B), and (C) are schematic views resulting from changes in temperatureand concentration; (A) represents an isotropic state, (B) represents atransparent nematic phase, and (C) represents a state (c) in whichnematic orientational orders have been locally formed.

However, although it is stated that, in such a liquid crystalmicroemulsion, the liquid crystals undergo phase transition in responseto changes in temperature and concentration to form a transparentnematic phase, there is no disclosure of an art to optically modulatethe transparent nematic phase. Furthermore, in the formation of saidemulsion (a), it is difficult to attain a microfine size and, moreover,many hours of standing results in separation of the transparent nematicphase and ceasing to maintain an emulsion state, so that there was roomfor further technical innovation.

Meanwhile, regarding a liquid crystal layer containing a dispersion ofparticles, there has been disclosed a liquid crystal display devicecomprising a liquid crystal element prepared by dissolving or dispersinga dichroic colorant and nanoparticles in a nematic liquid crystal andaligning liquid crystal molecules in a liquid crystal cell in adirection vertical to the substrates, with the provision for applicationof a voltage in the direction vertical to the substrates [for example,refer to Japanese Kokai Publication No. 2001-337351 (pages 1 to 3)]. Inthis liquid crystal display device, liquid crystal molecules are alignedin the direction vertical to the substrates to become transparent whenno voltage is applied, and under voltage application, nanoparticlesprevent alignment of dichroic colorant molecules etc. and the randomlyaligned dichroic colorant molecules absorb light to cause change inlight transmittance. It is to be understood that since the transparentstate in this liquid crystal display device is the state in which liquidcrystals are aligned in the direction vertical to the substrates, it isa state in which a general nematic phase is formed and not a state inwhich local nematic phases are formed.

Furthermore, there has been disclosed a liquid crystal optical elementcomprising a dispersion of inorganic oxide fine particles having arefractive index equivalent to the refractive index of a liquidcrystalline material either under application of a voltage or under novoltage application in a matrix of the liquid crystalline material [forexample, refer to Japanese Kokai Publication Hei-11-287980 (pages 1 to3)]. This liquid crystal optical element is designed to change in lighttransmittance by utilizing the refractive indexes of liquid crystallinematerial and inorganic oxide fine particles.

However, these are not devices utilizing the characteristic of atransparent nematic phase that despite the nematic orientational ordersare locally formed, an isotropic phase is macroscopically formed. Thus,there was room for a technical breakthrough to make a liquid crystaldisplay device improved in fundamental performance by using the abovecharacteristic of the transparent nematic phase.

Referring to a fine liquid crystal dispersion, an electrooptic materialcomprising a dispersion of liquid crystal droplets having particlediameters of smaller than 100 nm in a polyimide has been disclosed [forexample, refer to Japanese Kokai Publication Hei-11-249112 (pages 1 to3)].

This electro-optical material is produced by dispersing fine metalparticles or SiO₂ fine particles in a polyimide precursor, molding thedispersion into a film, and subjecting the film to a heating step andfurther to a step of dissolving the fine metal particles or SiO₂ fineparticles with a strong acid. However, said characteristic of atransparent nematic phase is not used even in such a fine liquid crystaldispersion.

Therefore, there has been a demand for a liquid crystal display of anovel mode which has the characteristic of a transparent nematic phaseand, at the same time, is capable of exhibiting this characteristic in astable manner to thereby exhibit various performance characteristicsnecessary for liquid crystal displays sufficiently.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above state of theart, and has for its object to provide a liquid crystal display devicein which a liquid crystal layer may form a stable transparent state and,in addition, which provides for a black display with an excellentviewing angle characteristic, as well as a high contrast ratio and ahigh response speed in liquid crystal display.

In the course of their research for developing a liquid crystal displaydevice of novel mode with improved viewing angle, response speed, andother performance characteristics, the present inventors paid attentionto the fact that in the isotropic state (A) in FIG. 6 which is the phasediagram of a liquid crystal microemulsion, a liquid crystal layerbecomes transparent and, therefore, by using a polarizing plate arrangedin cross nicol, for instance, an ideal black display may be obtained.The inventors' attention was further directed to the fact that even inthe transparent nematic phase (B), too, the liquid crystal molecules arenot arranged in the same direction similarly to said isotropic state,thus macroscopically the liquid crystal layer becomes a state same asthe isotropic state and, hence, the liquid crystal layer becomestransparent. It was found that in a system comprising a dispersion offine particles in a liquid crystal layer, the liquid crystal moleculesare isotropically aligned or become locally nematic state, that is tosay macroscopically the liquid crystal molecules become a randomlyarranged state or a state in which fine nematic phases are isotropicallydispersed, with the result that an optically isotropic state, that is tosay a transparent state, is obtained, and by using fine particles, thestable state can be made. The inventors found that if the liquid crystallayer constituting such a liquid crystal display device is opticallyisotropic when the voltage applied to the electrode is lower than athreshold value, but the arrangement of liquid crystal molecules isaltered to cause an optical transition due to optical rotation when theapplied voltage is not lower than the threshold value, improvements maybe realized in various characteristics such as viewing angle, contrastratio, and response speed characteristics, and the above-mentionedobject may be accomplished. They also found that in such a liquidcrystal display device, when the fine particles in the liquid crystallayer are provided with a surface treatment or they are as fine as 0.2μm or less in average particle diameter, the dispersion is stabilizedwithout undergoing phase separation for a long time, thus making itpossible to sufficiently inhibit non-uniformity on the display surfacedue to precipitation of particles leading to a locally unevendistribution of the fine particles. Therefore, when the proper kind offine particles is selected and the concentration of the fine particlesis judiciously controlled within the range of 1 to 20% by mass, cohesionof fine particles is inhibited, among other beneficial effects, to letthe device exhibit excellent fundamental performance characteristics.

They further found that even in a mode such that the fine particles inthe liquid crystal layer forms partition walls, with the liquidcrystalline material being surrounded by the microfine particles and, assuch, present in the form of droplets within the liquid crystal layer,that is the mode comprising liquid crystal droplets, the effects of thepresent invention are fully exerted. The present invention has beendeveloped on the basis of the above findings.

That is, the present invention relates to a liquid crystal displaydevice comprising an electrode and a liquid crystal layer betweensubstrates, wherein said liquid crystal layer consists of a liquidcrystalline material containing liquid crystal molecules and fineparticles, and is optically isotropic when the voltage applied to theelectrode is lower than a threshold value, and undergoes opticaltransition due to change in the arrangement of liquid crystal moleculeswhen the applied voltage is not lower than the threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing the state of theliquid crystal layer of the liquid crystal display device as anembodiment of the invention when the voltage applied to the transparentconductive film is lower than the threshold value.

FIG. 2 is a schematic cross-sectional view showing the state of theliquid crystal layer of the liquid crystal display device as anembodiment of the invention when the voltage applied to the transparentconductive film is not lower than the threshold value. The liquidcrystalline material used had a negative dielectric anisotropy, i.e.Δ∈<0.

FIG. 3 is a schematic view of the liquid crystal display device of FIG.1 as illustrated from the direction normal to the substrates.

FIG. 4 is a schematic view of the liquid crystal display device of FIG.2 as illustrated from the direction normal to the substrates.

FIG. 5 is a schematic cross-sectional view showing the state of theliquid crystal layer in the liquid crystal display device as anembodiment of the invention when the voltage applied to the transparentconductive film is lower than the threshold value, as illustrated fromthe direction normal to the substrates. The liquid crystalline materialused had a positive dielectric anisotropy, i.e. Δ∈>0.

FIG. 6 is a schematic view showing thetemperature/concentration-dependent phase diagram of the prior artliquid crystal microemulsion which appears in the prior art publicationreferred to hereinbefore. In the illustration, (A) represents anisotropic state, (B) a transparent nematic phase, and (C) a state inwhich nematic orientational orders are locally formed.

FIG. 7 is a schematic cross-sectional view showing the liquid crystaldisplay devices according to Examples 6 and 7 of the present invention.The liquid crystalline material used had a negative dielectricanisotropy, i.e. Δ∈<0.

FIG. 8 is a schematic cross-sectional view of the liquid crystal displaydevice according to Example 8 of the invention. The liquid crystallinematerial used had a positive dielectric anisotropy, i.e. Δ∈>0.

FIG. 9 is an enlarged cross-sectional view of a liquid crystal layercomprising nanosize liquid crystal droplets.

FIG. 10 is a schematic cross-sectional view showing a liquid crystaldisplay device of the structure comprising liquid crystal dropletsaccording to the present invention. The liquid crystalline material usedhad a positive dielectric anisotropy, i.e. Δ∈>0.

EXPLANATION OF NUMERALS AND SYMBOLS

-   12, 14: substrates-   13, 15: transparent conductive films-   18: slit-   20: liquid crystal layer-   22: liquid crystal molecules-   26: fine particles-   27: seal-   28: nanosize liquid crystal droplets-   29: nanosize partition wall formed by fine particles-   30: liquid crystal layer comprising nanosize liquid crystal droplets-   31: Codes-   a: emulsion-   c: state in which a nematic orientational order is locally formed

DETAILED DESCRIPTION OF THE INVENTION

The present invention is now described in detail.

The liquid crystal display device of the present invention comprises anelectrode and a liquid crystal layer between substrates. As thestructure of such liquid crystal display device, the formation positionof the electrode, the structure between the substrates, the process forforming the liquid crystal layer and the like are not particularlyrestricted. Generally, the liquid crystal layer is formed by filling aliquid crystalline material into the gap between the substrates. Forexample, the preferred embodiment is such that electrodes are formed onthe inner-facing side (internal surface) of a pair of spaced-apartsubstrates which are placed oppositely each other, and the liquidcrystal layer is formed by filling a liquid crystalline material into agap between the substrates or electrodes. The above structure includesthe form in which the electrode is formed on the inner-facing side ofeither substrate of the pair and the form in which the electrode isformed only on the inner-facing side of one of the substrates. In thismanner, a liquid crystal cell is formed and a liquid crystal displaydevice is produced.

In order that the substrates and electrode of the above liquid crystaldisplay device fully exert the functions as a liquid crystal displaydevice, transparent substrates and electrode are preferably used. As thetransparent electrode, for example, there may be mentioned a transparentelectrically conductive film, and the like. Desirably the gap betweenthe substrates is judiciously set so that the effect of the inventionmay be sufficiently exerted.

The liquid crystal layer mentioned above comprises a liquid crystallinematerial containing liquid crystal molecules and fine particles. Theliquid crystal molecules and fine particles are both comprised of one ortwo or more species. Moreover, it is preferable to disperse the fineparticles in the liquid crystalline material so that the liquid crystallayer may contain a dispersion of the fine particles. The liquidcrystalline material may contain additives such as an optically activesubstance and a dichroic dye. That is, insofar as the effect of thepresent invention can be exerted, the liquid crystal layer prepared fromsaid liquid crystalline material and fine particles may contain othercomponents in addition to said liquid crystal molecules and fineparticles.

The liquid crystal molecules mentioned above are not particularlyrestricted if they are capable of exhibiting liquid crystallinity, andfor example, nematic liquid crystal molecules are preferred. Thedielectric anisotropy (Δ∈) of the liquid crystalline materialconstituted by such liquid crystal molecules may be positive or negativebut the use of a liquid crystalline material with a negative dielectricanisotropy is preferred to the use of a liquid crystalline material witha positive dielectric anisotropy because of faster response speed andhigher aperture rate. This is because in the case where a liquidcrystalline material with a positive dielectric anisotropy is used, themode of changing a liquid crystal display by utilizing the movement ofliquid crystal molecules in the lateral direction (minor-axis direction)is the preferred mode but since such mode is constitutionally equivalentto the IPS mode, the response speed is not sufficient and, because ofthe immobility of liquid crystal molecules on the electrode, theaperture rate may also possibly be insufficient.

The dielectric anisotropy (Δ∈) mentioned above can be calculated by theequation of Δ∈=∈₁−∈₂, wherein ∈₁ represents the dielectric constant inthe direction of the long axis of the liquid crystal molecule and ∈₂represents the dielectric constant in the direction of the minor axis ofthe liquid crystal molecule. The dielectric constant ∈ can be calculatedby the equation of ∈=C_(p)d/S (wherein C_(p) represents the capacitanceof the liquid crystal; d represents the thickness of the liquid crystallayer; S represents the area of the overlapping part of electrodes ontwo substrates) as described in Okano, Mitsuharu & Kobayashi, Shunsuke(ed.): “Liquid Crystals, Fundamentals” (K. K. Baifu-Kan, Jul. 15, 1985,p. 215 to 216).

The liquid crystal layer in the present invention is optically isotropicwhen the voltage applied to the electrode is lower than the thresholdvalue and undergoes optical transition due to change in the arrangementof liquid crystal molecules when the applied voltage is not lower thanthe threshold value. The term “optically isotropic” used herein refersto a state in which optical properties are constant regardless of thedirection of measurement or, in the case of a liquid crystallinematerial, a condition which is evaluated is equivalent to that state.

The above liquid crystal layer may be such that it is more opticallyisotropic in the case where the voltage applied to the electrode islower than the threshold value as compared with the case in which theapplied voltage is not lower than said threshold value and that when theapplied voltage is not lower than the threshold value, the liquidcrystal molecules are arranged in a predetermined direction to cause achange in the optically isotropic state and thereby change the liquidcrystal display of the liquid crystal display device. In the presentinvention, it is preferable to set the case in which the voltage appliedto the electrode is lower than the threshold value to be the statecorresponding to no application of voltage to the electrode.

It is also preferable to arrange that when the liquid crystal layer isoptically isotropic, the light is not rotated but transmitted as suchthrough the layer and when the optically isotropic state has beenaltered, the light is rotated and the rotated light is transmittedthrough the liquid crystal layer. Furthermore, it is preferable that inthe optically isotropic state the light is not rotated but transmittedthrough the liquid crystal layer and the layer is transparent.

The threshold value mentioned above means the voltage value whichgenerates an electric field causing an optical change of liquid crystallayer and a change in the display on the liquid crystal display device.For example, when the transmittance in bright state is set at 100%, thethreshold value means the voltage value giving a transmittance of 10%.Additionally, in the present invention, the voltage value in brightstate is preferably set at not higher than 10V, and more preferably nothigher than 5V.

The fine particles for use in the present invention are not particularlyrestricted, and may be transparent or opaque. The above-mentioned fineparticles are preferably those causing vertical alignment of liquidcrystal molecules on the particle surface. The vertical alignment ofliquid crystal molecules on the surface of the fine particles means thealignment of liquid crystal molecules in the long axis direction on thesurface of the fine particles to form a nematic orientational orderaround the fine particles. In this state, liquid crystal molecules arealigned to surround the fine particles to form a mass. The size of themass varies with the surface aligning force of the fine particles andalso with different liquid crystalline materials.

In the present invention, the distance between the fine particles in theliquid crystal layer is preferably sufficiently shorter than thewavelength of visible light. Thus, it is preferable that the size ofabove-mentioned mass should not be larger than ¼ of the wavelength ofvisible light and that above-mentioned masses are densely filled in theliquid crystal layer. In this condition, the liquid crystal layer canexhibit a sufficiently transparent state.

Moreover, it is preferable that the above-mentioned fine particles havean average particle diameter of not more than 0.2 μm. If the averageparticle diameter of the particles exceeds 0.2 μm, the dispersingstability of the fine particles will not be sufficient and, moreover,the size of above-mentioned mass will not be small enough, i.e. notlarger than ¼ of the wavelength of visible light, with the result thatthe liquid crystal layer may possibly be deficient in optical isotropy.More preferred upper limit is 0.15 μm and more preferred lower limit is0.001 μm. The particle diameter distribution of such fine particles mayhave one peak or two or more peaks.

The average particle diameter mentioned above can be measured bywhichever of the following two methods.

(1) The method using a particle size distribution analyzer; theparticles are dispersed in a solvent and the average particle diameteris determined in the diluted solution. In this case, the measurement ispreferably carried out using an analyzer which enables overall profilingby yielding diameter-classified % by mass data. A preferred typicalmeasurement protocol comprises performing a measurement with theSuper-dynamic Light Scattering Spectrophotometer DLS-7000 (product name)manufactured by Otsuka Electronics Co., Ltd., which outputs the averageparticle diameter automatically, and using this output as the averageparticle diameter of the fine particles in the present invention.

(2) The method for measurement using a scanning electron microscope(SEM); the sample is photographed with a SEM at a suitable magnificationfor n=100 to 500, and the average value is used as the average particlediameter.

The material of the above-mentioned fine particles is not particularlyrestricted and may be organic fine particles or inorganic fineparticles, and, for example, organic solid particles, inorganic solidparticles, or the like can be used. The organic solid particles are notparticularly restricted if they are solid fine particles formed fromstyrenic or acrylic organic materials, etc. The inorganic solidparticles are not particularly restricted, either, if they are solidfine particles formed from inorganic material.

The term “solid particles” as used above means any fine particles to theexclusion of liquid fine particles. For example, fine particles formedby micelles in an emulsion such that either one of two mutuallyimmiscible liquid phases has been finely dispersed in the other phaseare excluded.

As the preferred form of the above-mentioned fine particles, there maybe mentioned (1) organic solid particles, (2) particles composed ofinorganic oxide, and (3) particles composed of fullerene and/or carbonnanotubes. Among these, it is preferable to use nanosize particles.

As the above-mentioned organic solid particles, it is preferable to use,for example, fine particles in the form of polymer beads such aspolystyrene beads, poly(methyl methacrylate) beads, poly(hydroxyethylacrylate) beads, divinylbenzene beads, etc. These may have beencrosslinked or uncrosslinked.

As the fine particles formed from inorganic oxide, for example, it ispreferable to use fine particles formed from silicon dioxide (SiO₂) orfine particles formed from metal oxide. Moreover, fine particlescomprising glass, silica, titania, alumina or other inorganic beads canbe preferably used. These fine particles may be hydrophilic orhydrophobic.

The fullerene mentioned above may be any of those having carbon atoms ina spherical shape. For example, one having a stable structure such thatthe number of carbon atoms is n=24 to 96. As such fullerene, the C60spherical closed shell carbon molecule consisting of 60 carbon atoms,among others, can be mentioned.

As the carbon nanotube mentioned above, for example, a cylindricalnanotube which is obtainable by circularizing the graphitic carbon-atomsurface of a layer having a thickness of several atoms can be used.

The shape of said fine particles is not particularly restricted andincludes spherical, ellipsoidal, block-shaped, columnar, and coneshapes, inclusive of these having protrusions or holes. The surface ofthe above-mentioned fine particles is not particularly restricted andmay for example be smooth or have irregularities, pits or grooves.

It is preferable that the fine particles are provided with a surfacetreatment. The surface treatment provided for the fine particles caneffectively stabilize a state of the liquid crystal layer. Examples ofthe surface treatment include a heat treatment and a provision of anorganic substance. Among them, the provision of an organic substance issuitable for the surface treatment. That is, it is preferable that thesurface of said fine particles is provided with an organic substance.The organic substance provided is not especially limited, but preferablyexhibits liquid crystallinity. For example, compounds represented by thefollowing structural formula (1) are preferable. Among them, a rigidliquid crystal core or mesogen each containing a cyclohexane ring,benzene ring, and the like is more preferable than a flexible connectiongroup constituted by a single bond such as alkane.

(l, m, n each representing an integer of 0 to 2.)

-   X; H, F, CN, CF₂H, CF₃, C₂F₅-   R and Y being dependent to each other and each representing the    following.-   R; alkyl containing 1 to 10 carbon atoms, alkoxy containing 1 to 10    carbon atoms, or hydroxyalkyl containing 1 to 10 carbon atoms-   Y; -, O, COO, OCO, NH, C═C, C═N, N═C, C≡C, NCON, CON, NOC-   Six-membered rings A to D each representing any of the following    functional groups.

(i representing an integer of 1 to 3. )

-   Z; -, C—C, C═C, C═N, C≡C

The provision of an organic substance may be chemical provision (bond)or physical provision (adhesion). For example, used is a method such asgrafting in which a polymer is chemically bonded to an inorganicsubstance surface.

And one or two or more surface treatments may be provided with the fineparticles.

Hereinafter, beneficial effects obtained by the provision of the surfacetreatment for the fine particles will be described with reference to“Introduction to Particle/Powder Technology” (written by TsubakiJyunichiro, Suzuki Michitaka, Kanda Yoshiteru, issued by NIKKAN KOGYOSHIMBUN Ltd., p. 104 to 105).

Fine particles in liquid including in liquid crystal diffuse in Browniandiffuse when the particle diameter of the fine particles becomes notmore than 1 μm and often have an electric charge. The charge state ofthe fine particles is represented by ζ (zeta) potential. Generally, iffine particles are dispersed in liquid and the fine particles havingelectric charges of the same code approach to each other, electrostaticrepulsion and van der Waals attraction act between the fine particles.And if the fine particles further approach to each other until theirelectron clouds overlap, strong repulsion also acts. As mentioned above,three forces all together act on the fine particles in the liquid. Thesmaller the particle diameter becomes, the more dominantly van der Waalsattraction acts. Therefore, the fine particles easily aggregation. Withthe above problem, mentioned is a method in which a surfactant is addedto emulsify and disperse the fine particles. However, the surfactant isan impurity in liquid crystal, and therefore has adverse effects such asdisplay defects during display on the liquid crystal display device.Therefore, energy making repulsion (dispersion force) high must beexternally given to the fine particles in order to emulsify and dispersethe fine particles in liquid crystal. That is, shearing force is neededfor the emulsion and dispersion of the fine particles in liquid. Thisprincipal of emulsion and dispersion (shearing force) is classified intothe one using external shearing force between fluid and contact surfaceby a media mill, a colloid mill, a roll mill, a pivoted method, or ahigh-pressure method, and the one using intermolecular force of fluiditself, that is, internal shearing force (solution-solution shearing)referred to as Reynolds stress produced by irregular variation ofvelocity in a turbulent flow region.

In the present invention, fine particles to be contained in liquidcrystal are provided with a surface treatment, such as provision of amodified group. Therefore, the fine particles can be dispersed withoutexternal energy generated by the above-mentioned large devices.

It is preferable that the content of the above-mentioned fine particlesis 1 to 20% by mass relative to the total mass of the fine particles andthe liquid crystalline material. Thus, the upper limit of fine particlecontent is preferably 20% by mass and the lower limit thereof ispreferably 1% by mass. If the particle content is less than 1% by mass,the low mixing ratio of the fine particles tends to prevent sufficientexpression of the effect of them. If it exceeds 20% by mass, theexcessively high formulating ratio tends to cause cohesion of particlesand consequent scattering of light. More preferable upper limit is 10%by mass and more preferable lower limit is 3% by mass. Theabove-mentioned total mass of fine particles and liquid crystallinematerial means the total amount of fine particles, liquid crystalmolecules, and any additives that may be formulated in the liquidcrystalline material.

In the liquid crystal display device of the present invention, it ispreferable that the above-mentioned liquid crystalline material has anegative dielectric anisotropy, i.e. Δ∈<0, and the above-mentionedliquid crystal display device is not subjected to a treatment foraligning the liquid crystal molecules.

Furthermore, it is preferable that the above-mentioned liquid crystaldisplay device is provided with a polarizing plate arranged in crossnicol on respective sides of the liquid crystal layer to yield a blackdisplay in optically isotropic state. Since a black display can thus beproduced in optically isotropic state, the black display is improved inquality (e.g. the transmittance of a black display is further reduced;the margin of thickness of the liquid crystal layer is increased; theviewing angle-dependent change decreases).

As such liquid crystal display device, for example, preferably, onewhich comprises transparent electrodes formed on the inner-facing sideof a pair of spaced-apart transparent substrates which are placedoppositely each other and a polarizing arranged in cross nicol, isoptically isotropic to become a dark state (black display) when thevoltage applied to the electrode is lower than a threshold value, andundergoes optical transition due to change in the arrangement of theliquid crystal molecules when the applied voltage is not lower than thethreshold value to become a bright state (white display). In this case,since the dielectric anisotropy (Δ∈) of the liquid crystal materialbecomes negative, i.e. smaller than 0, the longitudinal direction(long-axis direction) of the liquid crystal molecule moves to bevertical to the electric field generated between the transparentelectrodes formed on the inner-facing sides of the paired transparentsubstrates when the liquid crystal molecule is moved by applying avoltage between the substrates not lower than the threshold value.Thereby, it becomes possible to increase the response speed of theliquid display due to no use of the lateral direction (minor-axisdirection) of the liquid crystal molecules. Furthermore, by producing aliquid crystal display device which is not subjected to a treatment foraligning liquid crystal molecules comprised in the liquid crystalmaterial, it becomes possible to fully exert the state in which theliquid crystal molecules are irregularly arranged or the state in whichfine nematic phases are isotropically dispersed, coupled with theabove-mentioned effect of the fine particles in the liquid crystal layerwhen the voltage applied to the electrode is lower than the thresholdvalue.

The form in which said treatment for aligning liquid crystal moleculeshas not been carried out may for example be the form in which thesubstrates are neither provided with alignment layers nor subjected to arubbing treatment and the like.

As one preferred embodiment of the invention, there may be mentioned themode comprising liquid crystal droplets in the liquid crystal layerconstituting a liquid crystal cell. That is, it is preferable that theliquid crystal display device comprises the fine particles and theliquid crystal droplets surrounded by the above-mentioned fine particlesin the liquid crystal layer. Owing to the disposition of the fineparticles in the liquid crystal layer, for example, the inside of theliquid crystal layer is divided into minute-sized parts by the partitionwalls consisting of the fine particles to thereby form said liquidcrystal droplets. It is sufficient that at least one liquid crystaldroplet is present in the liquid crystal layer, and the shape, etc. ofeach liquid crystal droplet is not particularly restricted. As the fineparticles, any of those described above may be used. But as the fineparticles to be present around the liquid crystal droplet in suchmanner, those formed from inorganic oxide are preferred, and nanosizeparticles are preferable. Thereby, nanosize partition walls are formedwithin the liquid crystal layer.

It is preferable that the above-mentioned liquid crystal display devicehas a structure comprising laminated liquid crystal droplets. By virtueof such laminated structure of the liquid crystal droplets surrounded byfine particles, the effect of the liquid crystal droplets can besufficiently exhibited in the liquid crystal cell. That is, owing to thestable presence of the fine particles within the liquid crystal layer,the liquid crystal layer may form a stable transparent state and, inaddition, effects such as excellent viewing angle and contrast ratiocharacteristics in the liquid crystal display can be exerted. Thelaminated structure of the liquid crystal droplets may be a structuresuch that a plurality of liquid crystal droplets is present in avertical direction within the liquid crystal layer between thesubstrates constituting the liquid crystal cell.

It is preferable that the size of the above-mentioned liquid crystaldroplets is not larger than 200 nm. If it is larger than 200 nm, thelight may possibly scatter. The preferred range is not smaller than 50nm but not larger than 200 nm. While it is preferable that therespective liquid crystal droplets surrounded by the fine particlesshould fall within the above size range, it is permissible that amajority of maximum diameters of the respective liquid crystal dropletsfall within said range. The liquid crystal droplet within the above sizerange is referred to as “nanosize liquid crystal droplet” and the liquidcrystal layer formed by the droplets is referred to as “nanosize liquidcrystal droplet liquid crystal layer”.

The preferred liquid crystal display device according to the presentinvention is now described in detail, reference being made to theaccompanying drawings.

FIG. 1 is a schematic cross-sectional view showing the state of theliquid crystal display device of the present invention when the voltageapplied to the electrode is lower than the threshold value. FIG. 2 is aschematic cross-sectional view showing the state when the voltageapplied to the electrode is not lower than the threshold value.

The liquid crystal display device shown in FIG. 1 comprises a pair ofparallel substrates 12 and 14 made of glass or transparent resin, andtransparent conductive layers 13 and 15 disposed on the inner-facingsides of the substrates 12 and 14. The inner sides of these transparentconductive layers 13, 15 are neither provided with an alignment layernor have been subjected to a rubbing treatment. Interposed between thesetransparent conductive layers is a liquid crystal layer 20. The liquidcrystal layer 20 is composed of a liquid crystalline material comprisingnematic liquid crystal molecules and fine particles 26 as dispersedtherein.

In the above liquid crystal display device, the liquid crystal moleculesare disposed to line up in a certain settled direction when the voltageapplied to the electrode is lower than a threshold value. However,because the fine particle 26 has been dispersed in the liquid crystallayer as shown in FIG. 1, the liquid crystal molecules 22 are rathercontrolled by the orientation of the surface of fine particle 26.Usually, the surface orientation force of fine particles has the powerto align several liquid crystal molecules. Therefore, for example, theliquid crystal molecules 22 are aligned in the manner surrounding thefine particles 26, with one mass of the molecules being formed perparticle. The size of this mass is dependent on the orientation force ofthe fine particle surface and also on the species of liquid crystallinematerial. But it is preferable, as described above, that the size of themass should not be larger than ¼ of the wavelength of visible light andthat the liquid crystalline material should be densely filled with suchmasses. Thereby, when the liquid crystal layer 20 is viewedmacroscopically, it becomes a state in which the liquid crystalmolecules are irregularly arranged or a state in which fine nematicphases are isotropically dispersed, thus becomes a state which issufficiently isotropic optically.

In the case where a liquid crystalline material comprising nematicliquid crystal molecules and having a negative dielectric anisotropy,i.e. Δ∈<0, is used in the above-mentioned liquid crystal layer 20, boththe upper and lower substrates are preferably provided with slits 18 inthe formation of the transparent conductive layers. The cross-sectionalview of the liquid crystal display device under application of a voltagelower than the threshold value to the transparent conductive layer is asshown in FIG. 1, and the view from the normal direction of thesubstrates under application of a voltage lower than the threshold valueto the transparent conductive layer is as shown in FIG. 3. In this case,the liquid crystal molecules 22 are not aligned in the settleddirection.

When polarizing plates (not shown) or the like are used in this case,the liquid crystal layer can be made transparent when the voltageapplied to the transparent conductive layer is lower than the thresholdvalue. Thus, for example, by arranging the polarizing plates in crossnicol, a dark state (black display) can be created. Since, in thisliquid crystal display device, the liquid crystalline material becomes adark state (black display) in an optically isotropic state, the blackdisplay excellent in viewing angle characteristic can be obtained in theliquid display.

On the other hand, application of a voltage not lower than the thresholdvalue across the transparent conductive layers 13 and 15 generates anelectric field in the oblique direction to the substrate surface in theelectrode slit part (electrode edge part). Under the influence of thisoblique electric field, the direction of tilt of the liquid crystalmolecules 22 is decided so that, as shown in FIG. 2, the alignmentdirection of the liquid crystal molecules 22 is divided into the rightand left directions. The view from the normal direction of thesubstrates in the case where the applied voltage is not lower than thethreshold value is as shown in FIG. 4.

In this case, since the liquid crystal molecules 22 which are nematicliquid crystal molecules are aligned in the same direction as shown inFIGS. 2 and 4, that is to say an electric field is generated in thedirection oblique to the normal direction of the substrates, the liquidcrystalline material in the liquid crystal layer 20 forms a nematicphase, with the result that a bright state (white display) can becreated. The arrow marks in FIG. 4 schematically indicate the movementof liquid crystal molecules 22 in the longitudinal direction.

In the above embodiment, liquid crystal molecules are moved in thelongitudinal direction by applying a voltage not lower than thethreshold value across the transparent electrodes. Thus, the responsespeed can be made faster since lateral movement is not used and anelectric field in a direction oblique to the substrate surface isgenerated in the slit region of the electrode to decide the direction oftilt of the liquid crystal molecules.

When a liquid crystalline material comprising nematic liquid crystalmolecules and having a positive dielectric anisotropy, i.e. Δ∈>0, isused in the liquid crystal layer, a liquid crystal display device can befabricated by disposing a glass substrate patterned with a comb-shapedtransparent conductive layer at one side and a glass substrate notprovided with a transparent conductive layer at the other side as shownin FIG. 5. In this case, too, by utilizing polarizing plates (not shown)or the like means, the liquid crystal layer can be made transparent whenthe voltage applied to the electrode is lower than the threshold value.Thus, for example, by arranging the polarizing plates in cross nicol, adark state (black display) can be created. Moreover, application of avoltage not lower than the threshold value across the transparentconductive layers 13 and 15 causes the liquid crystal molecules 22,which are nematic liquid crystal molecules, to be aligned in the samedirection as shown in FIG. 5, with the result that a bright state (whitedisplay) can be created.

The liquid crystal display devices in FIGS. 7 to 10 are schematiccross-sectional views showing several embodiments comprising liquidcrystal droplets. FIG. 7 represents the case in which a liquidcrystalline material with Δ∈<0 is used; FIGS. 8 and 10 represent thecase in which a liquid crystalline material with Δ∈>0 is used; and FIG.9 is an enlarged cross-sectional view showing the liquid crystal dropletliquid crystal layer for constituting the liquid crystal layers of theseembodiments.

A liquid crystal cell of the above form can be fabricated by forming aninverse opal structure film on the substrate followed by conventionalpanel lamination and injecting (impregnating) with the liquidcrystalline material.

For example, a liquid crystal cell comprising liquid crystal dropletswithin a liquid crystal layer can be produced by a process whichcomprises (1) a step of mix-dispersing nanoparticles having averageparticle diameters of 50 to 200 nm which can be baked off at hightemperature (polystyrene, etc.) and nanoparticles having averageparticle diameters of not more than 5 nm which are not be baked off butconstitute partition walls of fine particles within the liquid crystallayer (SiO₂, etc.) in an aqueous solution to prepare a mixednanoparticle solution, (2) a step of dipping glass substrates havingtransparent electrodes with a slit in the above solution and forming afilm having a thickness of several μm on the substrates by pull-upmethod with utilizing the self-assembling phenomenon of the mixed fineparticles, (3) a step of baking said substrate at high temperature tobake off and gasify the above-mentioned nanoparticles having averageparticle diameters of 50 to 200 nm to leave holes at sites where theparticles had been present, and thereby form a film of inverse opalstructure on the substrate, (4) a step for laminating the substratestogether, and (5) a step of injecting (impregnating) the liquidcrystalline material to form liquid crystal droplets in the holes in theabove film having an inverse opal structure.

In the above embodiment, nanoparticles form partition walls within theliquid crystal layer between the substrates, and nanosize liquid crystaldroplets are formed in the holes surrounded by the nanoparticles. Inthis case, the following operational effects are exerted. That is,because the liquid crystal droplets are nanosize, no light scatteringtakes place in contrast with the polymer dispersed liquid crystal (PDLC)(If the nanosize exceeds 200 nm, a scattering may possibly occur).Moreover, since the size of the liquid crystal droplet is less than thewavelength order regardless of the alignment direction of liquid crystalmolecules in the liquid crystal droplet, no phase difference is shown(under no voltage application, optical isotropy is established withoutlight leakage).

Furthermore, when a liquid crystalline material comprising nematicliquid crystal molecules and having a positive dielectric anisotropy,i.e. Δ∈>0, is used as shown in FIG. 10, the transparent conductivelayers 13 (counter electrode) and 15 (source electrode) are formed onone of the substrates. By the voltage application, the voltage isapplied as indicated by the reference numeral 31 and accordingly theliquid crystal molecules in each liquid crystal droplet are aligned inone direction.

The liquid crystal display device of the present invention is notlimited to the above embodiments. For example, while the liquid crystaldisplay devices according to the above embodiments are transmission-modedevices, these may be provided with reflector means for use asreflection-mode devices or semi-transmission mode devices permittingselective use of the two modes.

The liquid crystal display device of the present invention, constitutedas above, has many advantages, namely the particle diameter of the fineparticles to be dispersed in the liquid crystal layer can be madesufficiently small to insure an improved dispersion stability of thefine particles and enable formation of a stable transparent state of theliquid crystal layer, giving an ideal black display with excellentviewing angle characteristics, a high contrast ratio, and a highresponse speed in liquid crystal display. Therefore, the device can beapplied to a variety of uses as a novel type of liquid display devicehaving excellent fundamental performance characteristics.

BEST MODE FOR CARRYING OUT THE INVENTION

The following examples are intended to illustrate the present inventionin further detail without defining the scope of the invention.

EXAMPLE 1

As the liquid crystalline material, MLC-2037 (product name; manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules and having anegative dielectric anisotropy, i.e. Δ∈<0, and as the fine particles,fullerene C60 of 99.9% by mass purity were used. The content of the fineparticles (the mixing ratio of the fine particles) based on the totalmass of the fine particles and liquid crystalline material was set to 5%by mass. This mixture was injected between the liquid crystal cellsfabricated using glass substrates each provided with a transparentconductive layer having slits and sealed to produce a liquid crystaldisplay device. The thickness of said liquid crystal cell was controlledto 5 μm. The liquid crystal cell was made to have the structure as shownin FIG. 1. In this fabrication process, the substrates were notsubjected to aligning treatments such as deposition of an alignmentlayer or rubbing. When polarizing plates were arranged in cross nicol,the liquid crystal layer became isotropic under no voltage applicationas shown in Table 1. The light transmittance was measured and found tobe 0.1%. Furthermore, when the voltage of 10V was applied between thetwo electrodes, the liquid crystal molecules are aligned in the samedirection, the light transmittance became 30%, and a sufficient contrastratio of 300 was obtained.

Moreover, the liquid crystal layer did not undergo phase separation evenafter being left for several days.

EXAMPLE 2

Except that the mixing ratio of the fine particles was changed to 15% bymass, a liquid crystal display device was fabricated in the same manneras in Example 1, and the light transmittance was measured.

As a result, a sufficient contrast ratio was obtained as shown in Table1.

Moreover, as in Example 1, the liquid crystal layer did not undergophase separation even after being left for several days.

Furthermore, when the mixing ratio of the fine particles was changed, itwas found that similar results can be obtained within the formulationrange of 1 to 20% by mass.

This device can also be provided with a reflecting means to be used as areflection mode device or a semi-transmission mode liquid crystaldisplay device.

EXAMPLES 3 AND 4

Except that the mixing ratio of the fine particles was changed to 0.5%by mass and 30% by mass, liquid crystal devices were fabricated in thesame manner as in Examples 1 and 2, and the light transmittance of therespective devices were determined. The results are shown in Table 1.

EXAMPLE 5

Using a liquid crystalline material comprising nematic liquid crystalmolecules and having a positive dielectric anisotropy, i.e. Δ∈>0, aliquid crystal display device was fabricated and the light transmittancewas determined. The liquid crystal cell was made to have the structureas shown in FIG. 5.

As the liquid crystalline material, MLC-6887 (product name; manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules was used. Theother parameters of cell thickness and fine particles were the same asin Example 1.

Also in this case, aligning treatments such as deposition of analignment layer or rubbing were not performed. When polarizing plateswere arranged in cross nicol, a black display could be obtained under novoltage application. When the voltage of 10V was applied between theelectrodes, a white display could be obtained as shown in FIG. 5.

The light transmittance was measured as in Example 1 and the result isshown in Table 1.

EXAMPLE 6

Except that silica powders having an average particle diameter of 200 nmwas used as the fine particles, a liquid crystal display device wasfabricated in the same manner as in Example 1, and the lighttransmittance was determined.

As a result, the contrast ratio tended to decrease as compared withExample 1, as shown in Table 1.

EXAMPLE 7

Except that silica powders having an average particle diameter of 400 nmwas used as the fine particles, a liquid crystal display device wasfabricated in the same manner as in Example 1, and the lighttransmittance was determined.

As a result, the contrast tended to decrease as compared with Example 1,as shown in Table 1. Thus, in Example 7, the particle scatteringincreased, and such contrast as obtained in Example 1 could not beobtained.

These results showed that the average particle diameter is preferablycontrolled to not more than 200 nm.

TABLE 1 Light Average transmittance Contrast particle Fine particle (%)ratio diameter (% by mass) 0 V 10 V (CR) Example 1 <50 nm 5 0.1 30 300CR high Example 2 <50 nm 15 0.1 30 300 CR high Example 3 <50 nm 0.5 2530 1.2 No appreciable effect of fine particles Example 4 <50 nm 30 1 2525 Cohesion of fine particles Example 5 <50 nm 5 0.1 25 250 — Example 6200 nm 5 0.2 25 125 — Example 7 400 nm 5 25 28 1.1 CR low(Method for Measuring the Transmittance)

Using LCD Evaluation System LCD-5000 (product name; manufactured byOtsuka Electronics Co., Ltd.), the voltage (frequency: 30 Hz or 60 Hz,waveform: rectangular wave) was applied to the cell and thetransmittance was measured. As reference standards, the condition of airwas set to 100% in the case of a transmission mode of display, and astandard white plate was set to 100% in the case of a reflection mode ofdisplay.

In these examples, it was found that the state in which liquid crystalmolecules are irregularly arranged or the state in which fine nematicphases are isotropically dispersed is exerted when the voltage appliedto the electrode is lower than the threshold value. When the devicesobtained in Examples 1, 2 and 5 showing excellent fundamentalperformance characteristics are compared with the device obtained inExample 3 comprising the fine particle mixing ratio of less than 1% bymass, since the mixing ratio of fine particles is low in Example 3, theeffect of the fine particles is not so appreciable and the nematic phaseaccounts for a major part of the liquid crystal layer. Thus, it wasfound that, in order to attain a sufficient black display withpolarizing plates arranged in cross nicol under no voltage application,the lower limit of mixing ratio of fine particles is preferably set to1% by mass. On the other hand, the device obtained in Example 4comprising the fine particle mixing ratio of exceeding 20% by mass, theexcessive content of fine particles resulted in cohesion of the fineparticles to cause light scattering under voltage application. Thus, inorder to obtain a sufficient contrast ratio, it was found that the upperlimit of mixing ratio of the fine particles is preferably set to 20% bymass.

In Examples 1, 2 and 5, it was found that since the mixing ratio of fineparticles was set within the preferred range, a sufficient black displaywas obtained with polarizing plates arranged in cross nicol under novoltage application, and a sufficient white display was obtained whenthe voltage applied to the electrode was not lower than the thresholdvalue, thus liquid crystal display devices excellent in contrast ratioand other fundamental performance characteristics could be obtained.

In addition, in Example 5, where the liquid crystalline material havinga positive dielectric anisotropy, i.e. Δ∈>0, was used, the lighttransmittance under voltage application of not lower than the thresholdvalue is decreased as compared with Examples 1 and 2 owing to theinability to control the liquid crystal molecules on the electrode.Thus, it was found rather preferable that a liquid crystalline materialof Δ∈<0 is used for the fabrication of liquid crystal display devices.

EXAMPLE 8

For the fabrication of a liquid crystal cell having nanosize liquidcrystal droplets, firstly, porous (inverse opal) layers having nanosizeholes were formed on the substrates. That is, in an aqueous solution inwhich fine polystyrene powders having a particle diameter of 200 nm andfine SiO₂ powders having a particle diameter of 5 nm were mixed anddispersed, glass substrates provided with a transparent electrode havingslits were dipped and then a layer having a thickness of several μm wasproduced by pull-up method with utilizing the self-assembling phenomenonof the mixed fine particles. Then, by baking at a high temperature togasify the polystyrene, a substrate with inverse opal structure having200 nm holes was obtained.

The substrates were laminated to form a cell, and a liquid crystallinematerial was injected into the holes to fabricate a liquid crystal cellhaving nanosize liquid crystal droplets with the structure shown in FIG.7.

As the liquid crystalline material, MLC-2037 (product name, manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules and having anegative dielectric anisotropy, i.e. Δ∈<0, was used.

In the course of fabrication, no aligning treatment, such as depositionof an alignment layer or rubbing, was carried out.

When polarizing plates were arranged in cross nicol, the liquid crystallayer became optically isotropic under no voltage application as shownin Table 2. The light transmittance was measured and found to be 0.1%.Furthermore, when the voltage of 10V was applied between the electrodes,the liquid crystal molecules were aligned in the same direction, thelight transmittance became 30%, and a sufficient contrast ratio of 300was obtained.

EXAMPLE 9

Except that the particle diameters of polystyrene were changed to 50 nmand 100 nm, liquid crystal display devices having nanosize liquidcrystal droplets filled with liquid crystal in 50 nm and 100 nmnanoholes were produced in the same manner as in Example 8, and thelight transmittance of each device was determined.

As a result, sufficient contrast ratios were obtained as shown in Table2.

Moreover, it was found that the transmittance under voltage applicationbecomes higher as the hole size becomes larger.

Conversely, it was found that the transmittance under voltageapplication becomes lower and the contrast becomes higher as the holesize becomes smaller.

This liquid crystal display device can also be provided with areflecting means to be used as a reflection mode or semi-transmissionmode liquid crystal display device.

EXAMPLE 10

Except that the particle diameter of polystyrene was changed to 400 nm,a liquid crystal display device comprising nanosize liquid crystaldroplets filled with liquid crystal in 400 nm nanoholes was produced inthe same manner as in Example 7, and the light transmittance wasdetermined.

As a result, as shown in Table 2, sufficient contrast ratios wereobtained for those having nanosize liquid crystal droplets of 50 nm(Example 9), 100 nm (Example 9)and 200 nm(Example 8), but a sufficientcontrast ratio was not obtained for that having nanosize liquid crystaldroplets of 400 nm.

EXAMPLE 11

Referring to the liquid crystal cell comprising nanosize liquid crystaldroplets, a liquid crystal display device was fabricated by using aliquid crystalline material comprising nematic liquid crystal moleculesand having a positive dielectric anisotropy, i.e. Δ∈>0, and the lighttransmittance was determined. The liquid crystal cell was made to havethe structure shown in FIG. 8.

As the liquid crystalline material, MLC-6887 (product name, manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules was used.

The other parameters of cell thickness and fine particles were the sameas in Examples 8 and 9.

Also in this case, aligning treatments such as deposition of analignment layer or rubbing were not carried out. When polarizing plateswere arranged in cross nicol, the liquid crystal layer became opticallyisotropic under no voltage application and a black display could beobtained, while a white display could be obtained under voltageapplication between the electrodes.

The light transmittance was determined as in Examples 8 and 9. Theresults are shown in Table 2.

TABLE 2 Liquid crystal nanosize Light transmittance (%) Contrast ratio(nm) 0 V 10 V (CR) Example 8 200 0.10 30 300 Example 9 100 0.05 25 50050 0.01 20 2000 Example 10 400 24 26 1.1 Example 11 50 0.01 20 2000

EXAMPLE 12

As the liquid crystalline material, MLC-2037 (product name; manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules and having anegative dielectric anisotropy, i.e. Δ∈<0, and as the fine particles,silica powders having a particle diameter of 20 nm, with which an alkylgroup (C₅H₁₁; in the above formula (1), l, m, n each represent 0, Xrepresents hydrogen, Y represents—(single bond)) was provided, wereused. The content of the fine particles (the mixing ratio of the fineparticles) based on the total mass of the fine particles and liquidcrystalline material was set to 5% by mass. Conventionally, a largedispersing device, in which a large amount of fine particles aredispersed with a high pressure device, is needed in order to dispersefine particles sufficiently. However, in this Example, use of a smallhomogenizer enables fine particles to disperse sufficiently into liquidcrystal, and no phase separation was observed after being left forseveral days. This mixture was injected between the liquid crystal cellsfabricated using glass substrates each provided with a transparentconductive layer having slits and sealed to produce a liquid crystaldisplay device. The thickness of the liquid crystal cell was controlledto 5 μm. The liquid crystal cell was made to have the structure as shownin FIG. 1. In this fabrication process, the substrates were notsubjected to aligning treatments such as deposition of an alignmentlayer or rubbing. When polarizing plates were arranged in cross nicol,the liquid crystal layer became isotropic under no voltage applicationas shown in Table 3. The light transmittance was measured and found tobe 0.1%. Furthermore, when the voltage of 10V was applied between thetwo electrodes, the liquid crystal molecules are aligned in the samedirection, the light transmittance became 30%, and a sufficient contrastratio of 300 was obtained.

EXAMPLE 13

Except that silica powders having an average particle diameter of 200nm, with which a compound represented by the following formula (2) wasprovided, were used as the fine particles, a liquid crystal displaydevice was fabricated in the same manner as in Example 1, and the lighttransmittance was measured.

As a result, a sufficient contrast ratio was obtained as shown in Table3.

Also in this Example, the small homogenizer was used to disperse thefine particles just like the fine particles provided with the alkylgroup in Example 12. Moreover, the liquid crystal layer did not undergophase separation even after being left for several ten days, because thefine particles in this Example were superior to the fine particlesprovided with the alkyl group in dispersibility.

Furthermore, when the mixing ratio of the fine particles was changed, itwas found that similar results can be obtained within the formulationrange of 1 to 20% by mass.

EXAMPLE 14

Except that the mixing ratio of the fine particles was changed to 15% bymass, a liquid crystal display device was fabricated in the same manneras in Example 12, and the light transmittance was measured.

As a result, a sufficient contrast ratio was obtained as shown in Table3.

Moreover, as in Example 12, the liquid crystal layer did not undergophase separation even after being left for several days.

Furthermore, when the mixing ratio of the fine particles was changed, itwas found that similar results can be obtained within the formulationrange of 1 to 20% by mass.

This device can also be provided with a reflecting means to be used as areflective or a transflective liquid crystal display device.

EXAMPLES 15 AND 16

Except that the mixing ratio of the fine particles was changed to 0.5%by mass and 30% by mass, liquid crystal devices were fabricated in thesame manner as in Example 12, and the light transmittance of therespective devices were determined. The results are shown in Table 3.

EXAMPLE 17

Using a liquid crystalline material comprising nematic liquid crystalmolecules and having a positive dielectric anisotropy, i.e. Δ∈>0, aliquid crystal display device was fabricated and the light transmittancewas determined. The liquid crystal cell was made to have the structureas shown in FIG. 5.

As the liquid crystalline material, MLC-6887 (product name; manufacturedby Merck Ltd.) comprising nematic liquid crystal molecules was used. Theother parameters of cell thickness and fine particles were the same asin Example 12.

Also in this case, aligning treatments such as deposition of analignment layer or rubbing were not performed. When polarizing plateswere arranged in cross nicol, a black display could be obtained under novoltage application. When the voltage of 10 V was applied between theelectrodes, a white display could be obtained as shown in FIG. 5.

The light transmittance was measured as in Example 12 and the result isshown in Table 3.

EXAMPLE 18

Except that silica powders having an average particle diameter of 200 nmwas used as the fine particles, a liquid crystal display device wasfabricated in the same manner as in Example 12, and the lighttransmittance was determined.

As a result, the contrast ratio tended to decrease as compared withExample 12, as shown in Table 3.

EXAMPLE 19

Except that silica powders having an average particle diameter of 400 nmwas used as the fine particles, a liquid crystal display device wasfabricated in the same manner as in Example 12, and the lighttransmittance was determined.

As a result, the contrast tended to decrease as compared with Example12, as shown in Table 3. Thus, in this Example, the particle scatteringincreased, and such contrast as obtained in Example 12 could not beobtained.

These results showed that the average particle diameter is preferablycontrolled to not more than 200 nm.

(Method for Measuring the Transmittance)

Using LCD Evaluation System LCD-5000 (product name; manufactured byOtsuka Electronics Co., Ltd.), the voltage (frequency: 30 Hz or 60 Hz,waveform: rectangular wave) was applied to the cell and thetransmittance was measured. As reference standards, the condition of airwas set to 100% in the case of a transmission mode of display, and astandard white plate was set to 100% in the case of a reflection mode ofdisplay.

TABLE 3 Light Average transmittance Contrast particle Fine particle (%)ratio diameter (% by mass) 0 V 10 V (CR) Example 12, 13 20 nm 5 0.1 30300 CR high Example 14 20 nm 15 0.1 30 300 CR high Example 15 20 nm 0.525 30 1.2 No appreciable effect of fine particles Example 16 20 nm 30 125 25 Cohesion of fine particles Example 17 20 nm 5 0.1 25 250 — Example18 200 nm  5 0.2 25 125 — Example 19 400 nm  5 25 28 1.1 CR low

In these examples, it was found that the state in which liquid crystalmolecules are irregularly arranged or the state in which fine nematicphases are isotropically dispersed is exerted when the voltage appliedto the electrode is lower than the threshold value. When the devicesobtained in Examples 12 to 14 and 17 showing excellent fundamentalperformance characteristics are compared with the device obtained inExample 15 comprising the fine particle mixing ratio of less than 1% bymass, since the mixing ratio of fine particles is low in Example 15, theeffect of the fine particles is not so appreciable and the nematic phaseaccounts for a major part of the liquid crystal layer. Thus, it wasfound that, in order to attain a sufficient black display withpolarizing plates arranged in cross nicol under no voltage application,the lower limit of mixing ratio of fine particles is preferably set to1% by mass. On the other hand, the device obtained in Example 16comprising the fine particle mixing ratio of exceeding 20% by mass, theexcessive content of fine particles resulted in cohesion of the fineparticles to cause light scattering under voltage application. Thus, inorder to obtain a sufficient contrast ratio, it was found that the upperlimit of mixing ratio of the fine particles is preferably set to 20% bymass.

In Examples 12 to 14 and 17, it was found that since the mixing ratio offine particles was set within the preferred range, a sufficient blackdisplay was obtained with polarizing plates arranged in cross nicolunder no voltage application, and a sufficient white display wasobtained when the voltage applied to the electrode was not lower thanthe threshold value, thus liquid crystal display devices excellent incontrast ratio and other fundamental performance characteristics couldbe obtained.

In addition, in Example 17, where the liquid crystalline material havinga positive dielectric anisotropy, i.e. Δ∈>0, was used, the lighttransmittance under voltage application of not lower than the thresholdvalue is decreased as compared with Examples 12 owing to the inabilityto control the liquid crystal molecules on the electrode. Thus, it wasfound rather preferable that a liquid crystalline material of Δ∈<0 isused for the fabrication of liquid crystal display devices.

1. A liquid crystal display device comprising an electrode and a liquidcrystal layer between substrates, wherein said liquid crystal layercomprises a liquid crystalline material containing liquid crystalmolecules and fine particles, and is substantially optically isotropicwhen voltage applied to the electrode is lower than a threshold valueand undergoes optical transition due to change in arrangement of theliquid crystal molecules when the applied voltage is not lower than thethreshold value, and wherein distance between the majority of the fineparticles in the liquid crystal layer is shorter than a wavelength ofvisible light.
 2. The liquid crystal display device according to claim1, wherein said fine particles are those causing vertical alignment ofthe liquid crystal molecules on the particle surface.
 3. The liquidcrystal display device according to claim 1, wherein said fine particleshave an average particle diameter of not more than 0.2 μm.
 4. The liquidcrystal display device according to claim 1, wherein said fine particlesare organic solid particles.
 5. The liquid crystal display deviceaccording to claim 1, wherein said fine particles are composed ofinorganic oxide.
 6. The liquid crystal display device according to claim1, wherein said fine particles are composed of fullerene and/or carbonnanotube.
 7. The liquid crystal display device according to claim 1,wherein said fine particles are provided with a surface treatment. 8.The liquid crystal display device according to claim 7, wherein thesurface of said fine particles is provided with an organic substance. 9.The liquid crystal display device according to claim 8, wherein saidorganic substance exhibits liquid crystallinity.
 10. The liquid crystaldisplay device according to claim 1, wherein the content of said fineparticles is 1 to 20% by mass relative to the total mass of the fineparticles and the liquid crystalline material.
 11. The liquid crystaldisplay device according to claim 1, wherein said liquid crystallinematerial has a negative dielectric anisotropy, i.e. Δ∈<0, and saidliquid crystal display device is not subjected to a treatment foraligning the liquid crystal molecules.
 12. The liquid crystal displaydevice according to claim 1, wherein said liquid crystal display deviceis provided with a polarizing plate arranged in cross nicol onrespective sides of the liquid crystal layer to yield a black display inoptically isotropic state.
 13. The liquid crystal display deviceaccording to claim 1, wherein said liquid crystal display devicecomprises fine particles and liquid crystal droplets surrounded by saidfine particles within the liquid crystal layer.
 14. The liquid crystaldisplay device according to claim 13, wherein said liquid crystaldisplay device has a structure comprising laminated liquid crystaldroplets.
 15. The liquid crystal display device according to claim 13,wherein the size of said liquid crystal droplets is not larger than 200nm.
 16. A liquid crystal display device comprising: an electrode and aliquid crystal layer between substrates wherein said liquid crystallayer comprises liquid crystalline material including liquid crystalmolecules and fine particles, and is substantially optically isotropicwhen voltage applied to the electrode is lower than a threshold valueand undergoes optical transition due to change in arrangement of theliquid crystal molecules when the applied voltage is not lower than thethreshold value, wherein distance between the majority of the fineparticles in the liquid crystal layer is shorter than a wavelength ofvisible light, and wherein the particles are at centers of respectiveclusters of liquid crystal molecules oriented therearound.
 17. Theliquid crystal display device according to claim 16, wherein said fineparticles are those causing vertical alignment of the liquid crystalmolecules on the particle surface.
 18. The liquid crystal display deviceaccording to claim 16, wherein said fine particles have an averageparticle diameter of not more than 0.2 μm.
 19. The liquid crystaldisplay device according to claim 16, wherein said fine particles arecomposed of inorganic oxide.
 20. The liquid crystal display deviceaccording to claim 16, wherein the content of said fine particles is 1to 20% by mass relative to the total mass of the fine particles and theliquid crystalline material.